Which of the following is a characteristic of Class A airspace?

Class A airspace is characterized primarily by its regulation of operations to ensure safety and efficiency in flight. It extends from 18,000 feet Mean Sea Level (MSL) up to and including 60,000 feet MSL. In this airspace, all aircraft are required to operate under Instrument Flight Rules (IFR), which means that pilots must be instrument rated and must fly according to rules that require adherence to ATC (Air Traffic Control) instructions. This ensures that there is a high degree of separation and traffic management due to the high density of air traffic and the altitude at which this class operates.

In Class A airspace, Visual Flight Rules (VFR) operations are not permitted. Pilots must file an IFR flight plan and maintain communication with ATC throughout the duration of their flight. This structured environment is essential for safety given the complexity of operations at these altitudes. The other options presented do not accurately reflect the requirements or characteristics of Class A airspace, such as allowing unrestricted flight or allowing VFR operations, which are prohibited in this class.